1. Field of the Invention
The present invention relates to a system for electronically controlling a petroleum production well and, more particularly, a system for intermitting the operation of the well in order to optimize the production efficiency from the well.
2. History of the Prior Art
Each underground hydrocarbon producing formation, known as a reservoir, has its own characteristics with respect to permeability, porosity, pressure, temperature, hydrocarbon density and relative mixture of gas, oil and water within the formation. In addition, various subterranean formations comprising a reservoir are interconnected with one another in an individual and distinct fashion so that the production of hydrocarbon fluids at a certain rate from one area of one formation will affect the pressures and flows from a different area of an adjacent formation.
Certain general characteristics are, however, common to most oil and gas wells. For example, during the life of any producing well, the natural reservoir pressure decreases as gases and liquids are removed from the formation. As the natural downhole pressure of a well decreases, the well bore tends to fill up with liquids, such as oil and water, which block the flow of the formation gas into the borehole and reduce the output production from the well in the case of a gas well and comprise the production fluids themselves in the case of an oil well. In such wells, it is also conventional to periodically remove the accumulated liquids by artificial lift techniques which include plunger lift devices, gas lift devices and downhole pumps. In the case of oil wells within which the natural pressure is decreased to the point that oil does not spontaneously flow to the surface due to natural downhole pressures, fluid production may be maintained by artificial lift methods such as downhole pumps and by gas injection lift techniques. In addition, certain wells are frequently stimulated into increased production by secondary recovery techniques such as the injection of water and/or gas into the formation to maintain reservoir pressure and to cause a flow of fluids from the formation into the well bore.
In oil and gas wells wherein the ambient reservoir pressure has been substantially depleted, two general techniques are commonly used: (1) plunger lift and (2) gas lift. In oil wells the goal of these techniques is to bring quantities of production liquids to the surface for collection while in the case of gas wells the goal is to clear the liquids from the well to allow the free flow of production gas from the well.
Plunger lift production systems include the use of a small cylindrical plunger which travels through tubing extending from a location adjacent the producing formation down in the borehole to surface equipment located at the open end of the borehole. In general, fluids which collect in the borehole and inhibit the flow of fluids out of the formation and into the well bore, are collected in the tubing. Periodically the end of the tubing is opened at the surface and the accumulated reservoir pressure is sufficient to force the plunger up the tubing. The plunger carries with it to the surface a load of accumulated fluids which are ejected out the top of the well. In the case of an oil well, the ejected fluids are collected as the production flow of the well, and in the case of a gas well the ejected fluids are simply disposed of thereby allowing gas to flow more freely from the formation into the well bore and be delivered into a gas distribution system at the surface. In the case of a plunger completion gas well, the production system is operated so that after the flow of gas from the well has again become restricted due to the further accumulation of fluids downhole, a valve in the tubing at the surface of the well is closed so that the plunger falls back down the tubing and is ready to lift another load of fluids to the surface upon the reopening of the valve. In the case of a plunger completion oil well, as soon as the plunger has reached the surface a valve in the tubing at the wellhead is closed so that the plunger also falls back down the tubing and is ready to lift another load of production fluids to the surface upon the accumulation of sufficient downhole casing pressure to lift the plunger and its load and the subsequent reopening of the valve.
A gas lift production system includes a valve system for controlling the injection of pressurized gas from a source external to the well, such as another gas well or a compressor, into the borehole. The increased pressure from the injected gas forces accumulated formation fluids up a central tubing extending along the borehole to remove the fluids as production flow or to clear the fluids and restore the free flow of gas and/or oil from the formation into the well. In wells where liquid fallback is a problem during gas lift, plunger lift may be combined with gas lift to improve efficiency. Such a system is shown in U.S. Pat. No. 4,211,279 issued July 8, 1980 to Kenneth M. Isaaks.
In each of the above cases, there is a requirement for the periodic operation of a motor valve at the surface of the wellhead to control either the flow of liquids and/or gas from the well or the flow of injection gas into the well to assist in the production of gas and liquids from the well. These motor valves are conventionally controlled by timing mechanisms and are programmed in accordance with principles of reservoir engineering which attempt to determine the length of time that a well should either be "shut in" and restricted from flowing gas or liquid to the surface and the length of time that the well should be "opened" to freely produce. Historically, the main criterion which has been used for the operation of the motor valve is strictly one of the elapse of a preselected time period. In some cases, measured well parameters, such as pressure, temperature, etc. are used to override the timing cycle under special conditions.
For example, U.S. Pat. No. 4,354,524 discloses a pneumatic timing system which improves the efficiency of using injected gas to artificially lift liquids to the wellhead by means of the plunger lift technique. U.S. Pat. No. 3,336,945 to Bostock et al. discloses a pneumatic timing device for use in timing the intermittent operation and/or injection of wells to increase the production. U.S. Pat. No. 4,355,365 to McCracken et al. discloses a system for electronically intermitting the operation of a well in accordance with timing techniques.
Other systems, such as the differential control system manufactured by Plunger Lift Systems, Inc. of Marietta, Ohio serve to operate a plunger lift completion in accordance with a gating system in which measured values of pressure and fluid level are compared with pre-set values. U.S. Pat. No. 4,150,721 to Norwood discloses a similar gas well controller system which also utilizes digital logic circuitry gating to operate a well in response to a timing counter and certain measured well parameters. U.S. Pat. No. 4,685,522 to Dixon et al. discloses a micro-processor based well production controller system Which monitors external parameters of the well and calculates values based upon an algorithm used to describe the performance of the well in order to control production from the well.
Under most circumstances, however, the mere timed intermittent operation of a single motor valve to control either outflow from the well or gas injection to the well will not effect maximum production nor will operation of the well based upon the comparison of well parameters with pre-set maximum and minimum values. This is primarily because the performance characteristics of the well are affected by a number of factors which continue to change over time. For example, the formation itself changes as production is taken from it so that the rate at which casing pressure builds up within the well to a value which is sufficient to cause the plunger to reach the surface also continues to change. Similarly, changes in the pressure of the output line from the well caused by a compressor or a production processing facility downstream, also cause pressure perturbations in the tubing of the well and affect the rate at which the plunger will rise to the surface of the tubing.
Other, more sophisticated, approaches to well production optimization have been used. For example, certain parameters associated with the producing well, such as casing pressure, tubing pressure, flow rate and pressure and oil/water mix, have been used as criteria upon which to base a decision as to when to intermittently open or close a well or when to intermittently inject fluids into the well to stimulate production of gas and/or liquids therefrom. These techniques have also encountered certain difficulties in that the changing parameters of the well cause the algorithms used to make the control decisions concerning intermitting of the well to become incorrect and no longer reflective of the well's performance.
An essential concept which must be taken into consideration when attempting the efficient intermitting of production flow from a well is that the characteristics of the well itself are extremely changeable things. A well is continually varying in its performance characteristics based upon both external and internal parameters so that it is virtually impossible to either program fixed time periods, as in the case with simple timed intermitter controllers, or to program in algorithms or parameter measurement based controls and have the well operate for a reasonable period of time without something in the well changing and altering the theory underlying the programming being used to attempt optimizing production from the well. In addition, the field within which the well is located also changes. Frequently production operations at other wells in the same reservoir or repair work on a separator within the gathering system to which the well is connected can cause a well to begin to load up and the preselected time periods of a timed intermitter will no longer be effective to optimize the controlled flow from the well.
A major factor to be considered in well operation is that throughout the intermitting of a well, the operator should guard against having the well "load up". This is a condition in which so much fluid is accumulated in the well bore that the maximum casing pressure of which the well is capable is insufficient to raise the plunger to the surface and purge the well of the accumulated fluids. Once a well loads up, it must be specially treated to remove the fluids from within the well and allow the intermitting process to begin again. Thus, if the well is not periodically "shut in" for a long enough period of time to allow sufficient downhole casing pressure to accumulate in order to raise the plunger all the way to the surface and completely clear the well when the valve at the wellhead is opened, it will require even greater casing pressure to do so the next time the valve is opened. The value of the accumulated downhole casing pressure is generally a direct function of the length of time during which the well is "shut in" before the surface valve is opened again.
When a well is manually intermitted, a well operator physically visits each well site on a periodic basis and either shuts the well in for a pre-selected period of time or opens the valve at the surface and allows the well to flow for a preselected period of time. In the mechanical timer operated intermitters, a mechanical device replaces the manual opening and closing of the valve by a timed opening and closing thereof. The operator simply selects the time period during which the well is to be shut in and the time period during which the well is to be allowed to flow and the intermitter automatically operates the valve. Experience over many years has shown that with both manual operation and timer controlled intermitters, operators generally select time periods which are relatively conservative with respect to optimizing the production flow from the well but which guard against the possibility of the well "loading up" and necessitating an expensive cleaning in order to place the well back into production again. In addition, operators tend to be distrustful of sophisticated electronic well optimizing equipment because they know from their experience even though there may be certain monitored parameters upon which intermitting of the well is based, the performance parameters of the well frequently change and thereby eliminate the accuracy with which the well is being operated. These inaccuracies introduce a risk of loading the well and the resultant negative reflection on the job performance by the operator which that brings.
A conservative approach to the intermitting of a well results in substantial waste of potential production capacity of the well. That is, in the case of a plunger completion oil well, the production flow from the well is directly related to the number of trips which the plunger makes from the bottom of the well to the wellhead in a given time period. Each time the plunger cycles and makes a round trip from the bottom, it delivers a slug of fluid as the production output from the well. Thus, it is desirable to allow the plunger to remain at the bottom only long enough to have the bottom hole pressure build to a value sufficient to raise the plunger all the way to the surface and complete a full cycle. Attempting to cycle the well too quickly results in the bottom hole pressure not building to a value large enough to raise the plunger all the way to the surface and its stopping its travel at some intermediate point and being unable to go further. This condition then requires the well to be again shut in. Failure of the bottom hole pressure to build to a sufficient value to clear the well the second time it is opened for flow runs the risk of loading the well and the required time and expense of swabbing the well before it can be again placed in production.
In the case of a plunger completion gas well, the quantity of production gas from the well is directly related to the length of time that the well can be left in an open and flowing condition without closing it in to cycle the plunger and clear accumulated fluids from the well to allow the free flow of gas from the well. Attempting to not leave the well closed for a sufficiently long period to build sufficient bottom hole pressure to raise the plunger all the way to the surface and fully clear the well of fluid again risks loading of the well and the cessation of production from the well until it has been cleared.
Because of the changing conditions within the well, in the reservoir within which the well is located, and in the external equipment connected to the output from the well, the rate at which the bottom hole pressure builds toward a value which is sufficient to cycle the plunger continues to vary throughout the life of the well. A controller which constantly evaluates the success with which the plunger is being repeatedly cycled and attempts to reduce the off-time while still successfully cycling the plunger would tend toward optimizing production from the well.
Moreover, it would be highly desirable to provide a programmable controller for the operation of a motor valve connected to a plunger completion well whereby the controller continues to reduce by small increments on each cycle the time that the well is shut in on each cycle in order to maximize the number of trips that the plunger is capable of making, for an oil well, or to maximize the gas flow time period for a gas well, both given the particular operating conditions of the well at any given time. Further, the controller should recognize when the off time for the well has been reduced to a value insufficient to fully cycle the plunger and compensate by increasing the off time during the next cycle by an incremental value sufficient to ensure the completion of the cycle. The system of the present invention provides such a programmable controller and method of well control for the optimization of well production while guarding against loading of the well.
The system of the present invention can be used in multiple applications for producing wells, for example, in any well which includes a cycling plunger such as gas lift completions, plunger lift completions, wells having fluctuating bottom hole pressures and production flow rates and, in addition, for the unloading or gas wells.